Re,
Si j'ai bien compris ta demande :
Option Explicit
'Option Private Module
Public Sub Convertir()
Dim ws As Worksheet
Dim lastRow As Long, rw As Long
Application.ScreenUpdating = False
Set ws = ActiveSheet
With ws
lastRow = .Cells(Rows.Count, 2).End(xlUp).Row
For rw = 7 To lastRow
If .Cells(rw, 2).NumberFormat = "-[hh]:mm" Then
' remplacer 10 par 2 après tests
.Cells(rw, 10).NumberFormat = "[hh]:mm"
.Cells(rw, 10) = .Cells(rw, 2) * -1
End If
Next rw
End With
Set ws = Nothing
End Sub